Traditionally, inhalers have been used to deliver medicament to the lung to treat local diseases of the lung such as asthma. However, when the inhaled particles are in the range 1 to 3 microns they can reach the deep lung area (alveoli) and cross into the bloodstream. This systemic delivery of pharmaceutically-active agents to the bloodstream via the lungs using an inhalation device has become a particularly attractive form of administering drugs to a patient many of whom are reluctant to receive drugs by injection using a needle. Furthermore, the administration of a drug using an inhaler may be carried out by a patient discreetly and in public without any of the known difficulties associated with injections involving a needle.
Although the repeatable dose of a powdered medicament emitted from a conventional dry powder inhaler may be relatively poor (the variation in respirable dose of known inhalers can be +/−20 to 30%), this is generally acceptable in the case of asthma drugs and the like. However, it will be appreciated that for the pulmonary delivery of systemic small molecule and protein and peptide drugs or for the administration of drugs such as insulin, growth hormone or morphine, this amount of variation in respirable dose is unacceptable. This is not only because it is considerably more important to ensure that the patient receives the same intended dose of these types of drugs each time the inhaler is used, so that a predictable and consistent therapeutic effect is achieved, but a relatively low respirable dose represents a significant wastage of what may be an expensive drug.
It will therefore be appreciated that for systemic pulmonary delivery, the provision of an inhalable aerosol requires an inhaler that can deliver the drug in a highly efficient, accurate and repeatable manner leading to a more predictable and consistent therapeutic effect which minimises any potentially harmful side effects for the patient as well as reducing the amount of costly drug required to deliver a therapeutic dose.
One important factor in maintaining the efficiency, accuracy and repeatability of the dose is to minimise the amount of drug that is retained in the inhaler mechanism and in the medicament pack in which the drug is stored prior to inhalation using the device. A conventional pack for an individual dose of dry powder medicament may include a gelatin capsule or a foil blister which is cold formed from a ductile foil laminate. A piercable foil laminate lid usually covers the blister which is heat sealed around the periphery of the blister. These types of package are preferred because each dose is protected from the ingress of water and penetration of gases such as oxygen in addition to being shielded from light and UV radiation and so offer excellent environmental protection. To administer a dose using a compressed gas powered inhaler, the capsule or foil lid is punctured by a piercing mechanism so that the drug can be entrained and carried to an aerosolising means, such as a nozzle, in a charge of gas which passes through the capsule or blister to the nozzle.
It will be appreciated that in an active inhaler of the aforementioned type, the same charge of gas provides the energy needed for both entraining the drug to evacuate the packaging and for aerosolising the drug once it has reached the nozzle. It is therefore important that the primary packaging does not present a significant restriction to the gas flow from the source of pressurised gas to the aerosolising nozzle. Bearing in mind that the amount of gas available for each dose is limited by what can be stored in a pressurised canister or generated in the device by the user by, for example, using a manually operated pump, the efficiency by which the drug is entrained in the airflow and so evacuated from its packaging must be as high as possible.
A problem with known inhalation devices in which the primary packaging for the drug is a conventional capsule or foil blister of the type referred to above is that it is possible for not all of the drug to be entrained in the airflow each time the device is used because the blister or capsule are typically pierced in such a way that the gas flowing into the blister through the pierced foil only partially scours the blister surfaces before flowing out of the blister. This problem is often exacerbated by the flap of foil cut by the piercing element as this can obscure parts of the blister from the flow of gas thereby restricting the free flow of gas throughout the entire volume of the blister and creating “dead” regions where gas flow is minimal or where secondary eddies form leading to powder becoming trapped. This trapped powder will have a significant detrimental effect on the repeatablility and accuaracy of the delivered dose as well as on the overall efficiency of the inhaler.
The efficiency of a dry powder inhaler may be measured in terms of the fine particle dose (FPD) or fine particle fraction (FPF). The FPD is the total mass of active agent which is emitted from the device following actuation which is present in an aerodynamic particle size smaller than a defined limit. This limit is generally taken to be 5 μm if not expressly stated as some alternative limit, such as 3μ or 1 μm, etc. The FPD is measured using an impactor or impinger, such as a twin stage impinger (TSI), multi-stage impinger (MSI), Andersen Cascade Impactor or a Next Generation Impactor (NGI). Each impactor or impinger has a pre-determined aerodynamic particle size collection cut points for each stage. The FPD value is obtained by interpretation of the stage-by-stage active agent recovery quantified by a validated quantitative wet chemical assay where either a simple stage cut is used to determine FPD or a more complex mathematical interpolation of the stage-by-stage deposition is used.
The FPF is normally defined as the FPD divided by the emitted or delivered dose which is the total mass of active agent that is emitted from the device following actuation and does not include powder deposited inside or on the surfaces of the device. The FPF may also, however, be defined as the FPD divided by the metered dose which is the total mass of active agent present in the metered form presented by the inhaler device in question. For example, the metered dose could be the mass of active agent present in a foil blister.